Experimental Investigation and Comparison of Bend Tube ......the maximum logarithmic mean...

International Journal of Scientific & Engineering Research, Volume 5, Issue 7, July-2014 ISSN 2229-5518

IJSER © 2014

http://www.ijser.org

Experimental Investigation and Comparison of Bend Tube Parallel & Counter Flow and Cross

Flow Water to Air Heat Exchanger Dipayan Mondal*, Md. Owal Ikram, Md. Fazla Rabbi, Md. Nawsher Ali Moral

Abstract—This present work represents the experimental investigation and comparison among the parallel flow, counter flow and cross flow

arrangements. A characteristic of heat exchanger design is the procedure of specifying a design, heat transfer area and pressure drops and checking

whether the assumed design satisfies all requirements or not. Here bend tube heat exchanger used for parallel and counter flow arrangements and

cross flow heat exchanger was chosen not only for occupying less space and better performance but also for comparing the performances. The primary

aim of this design is to obtain a high heat transfer rate without exceeding the allowable pressure drop. The type of design that is utilized determines the

coefficient of heat transfer and thus has an effect upon the surface area needed to obtain the desired level of heat exchange. The flow pattern through

most heat exchangers is a combination of counter flow, cross flow and parallel flow. But in bend tube heat exchanger, parallel flow and counter flow are

considered therefore counter flow is the most effective configuration for minimizing the needed heat transfer surface area. Within the experimental limit

the gain in temperature for parallel flow was to a maximum value of 120C and the maximum logarithmic mean temperature difference (LMTD), efficiency

and effectiveness were found 29.130C, 42.24% and 0.69 respectively. And for counter flow the gain in temperature was to a maximum value of 13

0C and

the maximum logarithmic mean temperature difference (LMTD), efficiency and effectiveness were 29.360C, 48.59%and 0.86 respectively. Again in cross

flow heat exchanger the gain in temperature was to a maximum value of 100C and the logarithmic mean temperature difference (LMTD) was found from

34.630C to 8.37

0C. The efficiency and effectiveness were found to maximum of 23.11% and 0.96 respectively.

Index Terms— Bend Tube, Parallel & Counter flow, Cross flow, Water to air heat exchange, Temperature distribution, Performances measurement

—————————— ——————————

1 INTRODUCTION

Heat exchanger is an equipment or device built for

efficient heat transfer from one medium to another,

whether the media are separated by a solid wall so that

they never mix, or the media are in direct contact. It

transfers heat from a hot fluid to a cold one [1]. In most heat

exchangers, heat transfer between fluids takes place

through a separating wall or into and out of wall in a

transient manner. In many heat exchangers the fluids are

separated by a heat transfer surface and ideally they do not

mix or leak. Such exchangers are referred to as indirect

transfer type heat exchanger and also referred to as surface

heat exchanger. The example of such heat exchanger is

automobile radiators [1-2]. In a few heat exchangers, the

fluids are in direct contact for exchanging heat. In the direct

contact heat exchangers, heat transfer takes place between

two immiscible fluids such as a gas and a liquid [1-6]. In

general if the fluids are immiscible, the separation wall may

be eliminated and the interface between the fluids replaces

a heat transfer surface as in a direct contact heat exchanger.

A heat exchanger consists of heat elements such as a core of

a matrix containing the surface and fluid distribution such

as headers manifolds, tank and inlet or outlet nozzle.

Usually there are no moving parts in a heat exchanger.

However there are exceptions such as a rotary regenerative

exchanger, scraped surface heat exchanger [3-9].

Heat exchangers are one of the most critical components in

any liquefaction/refrigeration system. Its effectiveness

governs the efficiency of the whole system. The major

requirement of these heat exchangers, working in the

cryogenic temperature range, is to have high effectiveness

[5]. In the recent past, Atrey has shown in his analysis that

decrease in heat exchanger effectiveness from 97% to 95%

reduces the liquefaction by 12%. The design of heat

exchangers, therefore, is very important from the system

performance point of view. The design should take various

losses, occurring during the exchange of heat and the

performance of the heat exchanger is governed by various

parameters like mass flow rates, pressures and

temperatures of working fluids etc. [16].

The simplest form of heat exchanger is the double-pipe heat

exchanger, which consists of a pipe that runs inside another

larger pipe. Types of double-pipe heat exchangers vary in

the number of tubes used and in the shape of those tubes.

A

————————————————

Author: Dipayan Mondal, Department of Mechanical Engineering,

Khulna University of Engineering & Technology (KUET), Khulna-

9203, Bangladesh, [email protected]

Co- Author: Md. Owal Ikram, Md. Fazla Rabbi, Md. Nawsher Ali

Moral , Department of Mechanical Engineering, Khulna University

of Engineering & Technology (KUET), Khulna-9203, Bangladesh

686

IJSER

mailto:[email protected]


IJSER © 2014


Concentric tube heat exchangers are the simplest type of

heat exchanger. The transfer of heat occurs between the

fluid that flows inside the smaller pipe and the fluid in the

space between the two pipes, through the surface of the

smaller pipe [4-9].

Many types of heat exchanger have been developed for use

at such varied levels of technological sophistication and

sizes as steam power plants, chemical processing plants,

petrochemical plants, petroleum refineries, and natural gas

processing, building heating and air conditioning and so

on. Common appliances containing a heat exchanger

include air conditioners, refrigerators, and space heaters.

These devices are also used in chemical processing and

power production [1-10]. In parallel flow arrangement, the

hot and cold fluids are flows through the same direction

but opposite in counter flow arrangement. In cross-flow

arrangement, the hot and cold fluids are flows at the right

angle to each other. Unlike a rotary heat exchanger, a cross-

flow heat exchanger does not exchange humidity and there

is no risk of short-circuiting the airstreams [1-2].

Fig.1: Arrangements for (a) Parallel flow; (b) Counter flow and (c) Cross flow.

2 RELATED WORKS

The following experimental works were done:

“Effect of Flow Arrangement on the Heat Transfer

Behaviors of a Microchannel Heat Exchanger” [11]

“Influence of Ionic Fluid in Parallel flow in Shell and Tube

Heat Exchanger” [12] “Performance evaluation of counter

flow heat exchangers considering the effect of heat in leak

and longitudinal conduction for low-temperature

applications” [13] “Analysis of a Counter Flow Parallel-

plate Heat Exchanger” [14] “Thermal performance analysis

of cross-flow unmixed-unmixed heat exchanger by the

variation of inlet condition of hot fluid” [15]

Nomenclature Dimensionless Parameters

D Diameter of the small tube (m) Re Reynolds number

d Diameter of the large tube (m) Nu Nusselt number

Dm Effective diameter (m) Pr Prandalt number

m Mass flow rate(Kg/s) F Correction Factor

K Thermal conductivity (W/m .0C)

Q Total heat transfer rate (W) Greeks

L Length of heat exchanger (m) Effectiveness of heat exchanger

A Surface area of heat transfer (m2) η Efficiency (%)

pC Specific heat of the fluid (J/kg. 0C) ρ Density of Fluid (kg/m3)

T Temperature (0C)

Tf Film temperature (0C) Subscripts

∆T temperature difference in heat exchanger (0C) h hot water

h Convective heat transfer coefficient (W/m2.0C) c cold air

U Overall heat transfer coefficient (W/m2.0C) i inside

ΔTln Logarithmic Mean Temperature Difference (LMTD) 0C o out side

687

IJSER


IJSER © 2014


3 MATERIALS AND DESIGN CRITERIA

3.1. Materials used and stepwise construction

In bend tube heat exchanger, firstly Galvanized Iron pipe

were cut into small pieces and the copper tube were bended

using sand and inserted concentrically into the Galvanized

Iron pipe. Then the outer Galvanized Iron pipe was welded

with Elbow and T for the bended area. Again for cross flow

heat exchanger, copper tube was bended using sand and

then the copper tube was placed into the wooden frame.

Galvanized Iron pipe was holed by 1.5mm drill bit. Then it

was hanged up along the copper tube surrounding the

sheet metal with the wooden frame. Gate valve and blower

were used to control the mass flow rate of hot water and

cold air to the pipe on the both cases. Separate

thermocouples are used on both for measuring the hot

water and cold air temperatures with the digital meter.

3.2. Design Condition

The design conditions are usually specified for estimating

heat transfer between inside and outside. It was desired to

determine the exit temperatures of the fluids for various

entrance conditions. Particular set of conditions depends on

many factors other than heat transfer aspects- like cost,

space requirements, personal opinions of the designer etc.

3.3. Selection of Fluid

In bend tube heat exchanger, two kinds of fluid both of

unmixed are used in where the cold fluid, supply air passes

throw the galvanized iron pipe and at every point it gains

heat. The hot water enters through the cupper pipe of heat

exchanger from the hot water tank and at every point it

losses heat and finally it leaves the tube at a certain lower

temperature [1-10].

Again in cross flow heat exchanger, two kinds of fluid both

of unmixed are used in where the cold fluid, supply air

passes inside the copper tube and at every point it gains

heat. The supply hot water enters the galvanized iron pipe

at a certain higher temperature and exit the pipe at a certain

lower temperature [1-10].

3.4. Governing Equation

The total amount of heat transfer is denoted

by; pmCQ ; Where, m is total mass flow, kg.s-1; pC is

the specific heat of the fluid, J.kg-1. 0C -1; ∆T is the

temperature difference in heat exchanger, 0C [1].

The overall heat transfer coefficient U is calculated with the

following relations [6];ln

UAQ ; Where Q =Total heat

transfer (W); U = Overall heat transfer coefficient

(W/(m²·0C)); A = Heat transfer surface area (m2); ΔTln = log

mean temperature difference (0C).

Heat transfer for pulsating flow in a curved pipe was

numerically studied by Guo et al. for fully developed

turbulent flow for the Reynolds number range of 6000 to

18000 [1]. The Nusselt No. is given below; 4.058.0 PrRe328.0Nu

But now Dittus-Boelter correlation [1] is

used;4.08.0 PrRe023.0Nu .The heat transfer

coefficient (h) [6] is calculated from the relation

below;k

DhNu m

; where 0DdD im is mean effective

diameter of larger pipe.

The LMTD is calculated from the expression [7-10];

e

i

ei

ln

The effectiveness is calculated from the following relations;

Effectiveness for the parallel flow [1],

)max

min1(

)]max

min1)(

min

exp[(1

C

C

C

C

C

UA

Effectiveness for counter flow [8],

)]1)(exp[()(1

)]1)(exp[(1

max

min

minmax

min

max

min

min

C

C

C

UA

C

C

C

C

C

UA

Again, for forced convection and flow inside the cylinder,

Nusselt number is given by4.08.0 PrRe023.0Nu .This

equation is called Dittus-Boelter equation which can be

used only when 10000Re [1-10].

Again for forced convection and flow over the cylinder,

Nusselt number is given

by 4.032

5.0 Pr)Re06.0Re04.0( Nu

This equation is called Whitaker correlation which can be

used only when 100000Re40 [1].

688

IJSER


IJSER © 2014


The heat transfer coefficient (h) [6] is calculated

from;k

DihNu

; where, Di=diameter of copper tube.

The LMTD is calculated from the expression [5];

e

i

ei

ln

3.5. Design Criteria

Table 1: Heat Exchanger design parameters

Design Parameters Bend tube Heat Exchanger Cross flow Heat Exchanger

Small tube Inside& Outside diameters

(Material: copper tube)

mDi 0127.0 & mD 014.00 mDi 0127.0 & mD 014.00

Large tube Inside& Outside diameters

(Material: GI pipe)

mdi 0381.0 & md 045.00 mdi 0381.0 & md 045.00

Hot water & cold air mass flow rate skgmh /027.0 & skgmc /024.0 skgmh /027.0 & skgmc /00564.0

Inlet and outlet temp. of hot water CTh0

1 80 & CTh0

2 60 CTh0

1 80 & CTh0

2 65

Inlet and outlet temp. of cold air CTc0

1 27 & CTc0

2 40

CTc0

1 30 & CTc0

2 40

Number of holes on the GI pipe 3 Row each of 18 holes and

Diameter of the each hole m0015.0

For Bend tube Heat Exchanger: On the basis of the film

temperature of water; 45.65624

i

he

D

mR

Dittus-Boelter correlation; 56.37PrRe023.0 4.08.0 Nu &

k

DhNu ii

; and then find Cmwhi

02 ./61.1986

Again for cold air; mDdD im 0241.00 and then

56.635564

m

ce

D

mR


k

DhNu m

0

; and then find Cmwho

02 ./49.154

Now,Cmw

hihiD

DU 02 ./30.142

0

110

1

&

C

e

i

ei 039.36

ln

Again, wTTCmQ hhphh 789.2262)( 21 and using

Correction factor, 96.0F with the help

of ln0ln )( ULDFFAUQ it is found mLL s 35.10

For Cross flow Heat Exchanger: On the basis of film

temperature of water; if drill bit diameter is d then,

2

22/27.1

0015.077.97912

027.04

12

4sm

d

mu

& 15.3921610421.0

013.027.16

ce

DuR

Whitaker correlation; 4.03

25.0 Pr06.0Re04.0

RNu &

k

DihNu i

; and then find Cmwhi

02 ./02.76580

Again for cold air; 85.305944

i

ce

D

mR


k

DhNu i

0

; and then find Cmwho

02 ./98.160

Now,Cmw

hih

U 02 ./48.154

0

11

1

&

C

e

i

ei 044.37

ln

Again, wTTCmQ hhphh 09.1697)( 21 and using

Correction factor, 96.0F with the help

of lnln )( ULDFFAUQ c it is found mL 51.7 ; A

suction type air blower was used for the both cases having

capacity rpmhp 5200,1

689

IJSER


IJSER © 2014


Fig. 2. Schematic diagram (a) for parallel flow; (b) for counter flow (water flow direction is reversed of fig. a); (c) for cross flow heat exchanger.

4 RESULTS AND DISCUSSIONS

Under the steady condition, data were collected and recorded and hence the mass flow rate of hot water was varied and the

mass flow rate of cold air was fixed.

Fig.3: Temperature distribution curves for parallel flow for (a) observation 1 to 2; (b) observation 3 to 5; (c) observation 6 to 8.

From the above representation for parallel flow, it is observed that the temperature of hot water was varied from 750C to 490C

and the cold air temperature was varied from 290C to 270C and it was also observed to pass total length, the hot water

temperature was decreased from 580C to 380C and the air temperature was increased from 410C to 370C.

690

IJSER


IJSER © 2014


Fig. 4: Temperature distribution curves for counter flow for (a) observation 1 to 2; (b) observation 3 to 5; (c) observation 6 to 8.

Fig. 5: Temperature distribution curves of related work for counter flow

[14].

Again for counter flow from the above figure, it is observed

that the temperature of hot water was varied from 740C to

490C and the cold air temperature was varied from 290C to

270C and it was also observed to pass total length, the hot

water temperature was decreased from 550C to 360C and

the air temperature was increased from 350C to

410C.Whatever the nature of temperature profiles almost

same both of this project and the referenced work.

691

IJSER


IJSER © 2014


Fig. 6. (a to g)Temperature distribution curve for observation 1 to 7

From the representation for cross flow arrangements, it is

observed that during flow the temperature of hot water

was varied from 750C to 480C and the cold air temperature

was varied from 300C to 280C and it was also observed that

to pass the entire length of copper tube, the hot water

temperature was decreased from 650C to 370C and air

temperature was increased from 330C to 380C.

Presentation of result for parallel flow, counter flow and for cross flow arrangements remaining constant mass flow rate of cold

air, mc=0.0238 kg/s and mc=0.01 kg/s respectively for bend tube and cross flow heat exchanger.

Fig.7: Performance curves for (a) parallel flow; (b) counter flow (c) cross flow arrangements.

692

IJSER


IJSER © 2014


From the above graphical representation for parallel flow,

the mass flow rate of hot water varied from 0.0138 kg/sec to

0.0061 kg/sec and the measured effectiveness was varied

from 0.69 to 0.51; LMTD was varied from 29.130C to 6.790C;

efficiency was varied from 42.24% to 31.53%. Whereas for

counter flow, the mass flow rate of hot water varied from

0.0124 kg/sec to 0.0060 kg/sec and the measured

effectiveness was varied from 0.86 to 0.74; the LMTD was

varied from 29.360C to 11.320C, efficiency was varied from

48.59% to 42%. Again for cross flow arrangement, the mass

flow rate of hot water varied from 0.012 kg/sec to 0.0061

kg/sec and mass flow rate of cold air kept constant at 0.01

kg/sec and it is observed that the calculated LMTD was

varied from 34.630C to 10.870C, efficiency was varied from

18.95% to 13.6% and effectiveness was varied from 0.96 to

0.86.

Fig.8: Comparison of the performances among the parallel flow, counter flow and cross flow arrangements.

From the above cure it is concluded that the observed LMTD, efficiency and the effectiveness were maximum for the counter

flow arrangement for the desired heat exchanger.

Fig.9: Comparison the Relationship between the heat flux and the mass flow rates (a) for this work and (b) for the referenced work [11].

It is seen that the heat flux increases with the increase of mass flow rate. In our project the heat flux is measured more for the

counter flow than the parallel flow arrangements. These are the almost same character of the related work.

(b) (a)

693

IJSER


IJSER © 2014


(a) Our Work (b) Related Work [15]

Fig.10: A comparable relationship for cross flow arrangement between our work and the related work [15].

From the above fig.10, it is seen that not only the efficiency

but also the heat transfer rate are increased with the

increase of the mass flow rate of hot fluid but a reverse

direction was found at first on the both of efficiency and

heat transfer rate analysis. The initially decreasing

phenomena are measured in our work but the decreasing

phenomena at last segment were measured at the related

work.

It is concluded that not only in parallel flow but also in

counter flow the temperature of hot water was decreased

with distance almost uniformly and the temperature of cold

air was increased with distance. But the difference between

the parallel and counter flow characteristics was that the

temperature difference was more rapid and effectiveness is

also greater in counter flow than that of parallel flow.

Again it is found from the competitive study the counter

flow arrangement also gives the greater temperature

difference; heat flux and the effectiveness than that of cross

flow arrangement.

The designed value that was assumed was not obtained

during experiment. It was deviated. The mass flow rate of

hot water was not maintained as the assumed value due to

the low head tank. The mass flow rate of air was obtained

by calibrating with previous data which was not

maintained as the assumed value. In the bend tube heat

exchanger the hot water is passed through the copper pipe

which was not always maintained the concentric with

Galvanized iron pipe and for the cross flow arrangement

the hot water was flow over the copper tube from the

galvanized iron pipe which was not contacted all the

portion of the copper tube due to the alignment problem.

So the temperature difference was not obtained as the

assumed value. And the result of this experiment was

fluctuated from the design result.

5 CONCLUSION

On the basis of the experimental the gain in temperature for

parallel flow was to a maximum value of 120C, for water

flow rate of 0.0128 kg/sec and air flow rate of 0.0238 kg/sec.

Within the experimental limit the gain in temperature for

counter flow was to a maximum value of 130C, for water

flow rate of 0.0077 kg/sec and air flow rate of 0.0238 kg/sec.

Within the experimental limit LMTD was found from 150C

to 300C.The efficiency and the effectiveness were found to a

maximum value of 48.59% and 0.86 respectively. Again for

cross flow arrangement, the gain in temperature was to a

maximum value of 100C, for water flow rate of 0.014 kg/sec

and air flow rate of 0.01 kg/sec. Within the experimental

limit LMTD was found from 34.630C to 8.370C.The

efficiency, effectiveness and the LMTD were found to a

maximum value of 23.11%, 0.96 and 34.630C respectively.

Overall heat transfer coefficient was found to a maximum

value 157.67 w/m2. 0C

694

IJSER


IJSER © 2014


ACKNOWLEDGMENTS

The authors wish to thank Prof. Dr. Md. Nawsher Ali

Moral, Department of Mechanical Engineering, Khulna

University of Engineering & Technology (KUET) and

would also like to gratefully KUET, Bangladesh for the

financial support.

REFERENCES

[1] Ozisik, M.N; “Heat Transfer, A Basic Approach.” International Edition, 1985; Tata McGraw-Hill Publishing Company Limited; New Delhi W.-K. Chen, Linear Networks and Systems. Belmont, Calif.: Wadsworth, pp. 123-135, 1993. (Book style)

[2] http://1heatexchanger.info/2009/10/what-is-a-heat-

exchanger-2/

[3] http://www.engineersedge.com/heat_transfer/parallel_counter_flow_designs.htm

[4] http://en.wikipedia.org/wiki/Heat_exchanger/Plate_heat_exchanger

[5] Kays, W.M., and A. L. London: Compact Heat

Exchangers, 2nd edition, McGraw-Hill, New York, 1964

[6] http://en.wikipedia.org/wiki/Heat_transfer_coefficient [7] http://en.wikipedia.org/wiki/Log_mean_temperature_d

ifference [8] http://en.wikipedia.org/wiki/Effectiveness [9] Nag, P.K; “Heat and Mass Transfer,” Second Edition;

Tata McGraw-Hill Publishing Company Limited; New Delhi

[10] Holman, J.P; “Heat Transfer”, Ninth Edition, 2004; Tata McGraw-Hill Publishing Company Limited; New Delhi

[11] Dang Thanhtrung, Teng Jyh-tong, and Chu Jiann-

cherng; “Effect of Flow Arrangement on the Heat

TransferBehaviors of a Microchannel Heat

Exchanger”,Proceeding of the International Conference

of Engineers & Computer Scientists 2012 Vol III,

IMECS 2010,March 17-19, Hon Kong, ISBN: 978-988-

18210-5-8.

[12] N.D.Shirgire1 S.S.Bhansali, Dr. U.V.Kongre,

P.S.Ghawade, P.R.Bodade;“Influence of Ionic Fluid in

Parallel flow in Shell and Tube Heat Exchanger”,

International Journal of Engineering Science and

Innovative Technology (IJESIT) Volume 2, Issue 3, May

2013, ISSN: 2319-5967, ISO 9001:2008 Certified

[13] Prabhat Gupta *, M.D. Atrey; “Performance evaluation

of counter flow heat exchangers considering the effect

of heat in leak and longitudinal conduction for low-

temperature applications”, Cryogenics 40 (2000) 469-

474, Cryogenics and Superconductivity Section, Centre

for Advanced Technology (CAT), Indore 452 013, India,

Received 4 September 2000; accepted 13 November

2000

[14] Ruoxu Jia, Junling Hu, and Abubaker E.M Elbalsohi;

“Analysis of a Counter Flow Parallel-plate Heat

Exchanger”, ASEE 2014 Zone I Conference, April 3-5,

2014, University of Bridgeport, Bridgeport, CT, USA.

[15] Dr. M K Chopra and Ramjee Singh Prajapati; “Thermal

performance analysis of cross-flow unmixed-unmixed

heat exchanger by the variation of inlet condition of hot

fluid”, International Refereed Journal of Engineering

and Science (IRJES), Volume 3, Issue 1 (January 2014), PP.

29-31, ISSN (Online) 2319-183X.

695

IJSER

Date post:	09-Mar-2021
Category:	Documents
Upload:	others
View:	2 times
Download:	0 times

Experimental Investigation and Comparison of Bend Tube ......the maximum logarithmic mean...

Documents